Electrical circuit test probe and connector

ABSTRACT

An improvement to electrical circuit test probes of the type wherein a socket tube having a cylindrical spring loaded plunger therein is mounted in a test fixture. Electrical contact is assured between the socket and the plunger as the plunger moves longitudinally by the plunger having a resiliently flexible wand extending inwardly therefrom at an angular offset to the longitudinal axis of the plunger and the wand terminating in a bulbous contact member. The contact member is disposed within a cylindrical plunger tube and passes through an opening through which the contact member cannot pass. As a result, the wand acts as a flexible resilient finger holding the contact member firmly against the inner wall of the plunger tube as the plunger is moved in and out through its limits of movement.

BACKGROUND OF THE INVENTION

The present invention relates to electrical circuit test probes andelectrical connectors therefor and, more particularly, to an electricalcircuit test probe with a spring loaded plunger in a socket tube havingan offset wand extending therefrom to contact the interior of a memberdisposed within the socket tube and wherein the socket tube is providedwith a releasable connector for a conductor comprising a plug housingformed in the end of the socket tube and a collet jawed plug forgripping the electrical conductor and slidably fitting within the plughousing to grip the conductor and hold it therein in electrical contact.

Of necessity, electrical circuit testing has made rapid strides in itstechnology in the past few years. Until the advent of miniaturization,circuit testing, took the form shown in FIG. 1. To test whether a largefuse 10 was electrically conductive, a pair of large, hand-held probes12 connected to a meter 14 were placed across the fuse 10.

Modern printed circuit (PC) boards have made such hand testing virtuallyimpossible. Literally hundreds of connections at close-spaced proximitymust be tested to verify proper circuit continuity. To do this in anautomated way, apparatus such as that shown in FIG. 2 has come intocommon usage. The PC board 16 is positioned on a holding table 18 and afixture 20 having a plurality of electrical test probes 22 is broughtinto contact with the surface thereof. The probes 22 are connected bycable 24 to testing logic 26. In this manner, the testing logic 26 canquickly and accurately test a multitude of circuit interconnections forproper continuity.

As can be readily understood, the functioning of the probes 22 relativeto the fixture 20 and their electrical connection to the testing logic26 are critical to the successful operation of the testing apparatus.Any failure of the probe 22 to make effective electrical contact withthe PC board 16 or any failure of the electrical contact so made to beeffectively passed back to the testing logic 26 will be interpreted,wrongfully, by the testing logic 26 as a failure of the PC board 16.

Two aspects are critical in the overall operation of the probes 22.First, the probes 22 must have a degree of linear axial movement withinthe fixture 20 to accommodate variations in projection of contact pointsfrom the PC board, thereby to prevent undue pressure on the PC board 16and/or breakage of the probe 22 while ensuring adequate electricalcontact. Second, is the manner of electrically connecting a conductorfrom the cable 24 to the probe 22. Some of these aspects according tothe prior art are shown in FIGS. 3-8.

Turning first to FIG. 3, a probe is shown according to U.S. Pat. No.4,200,351. The probe, generally indicated as 28, comprises aspring-metal tube 30 press-fit within bore 32 of fixture 20. Probe 28has a first cylindrical portion 34 adapted to slidably fit within thetube 30 and having a head 36 on the outer end thereof for contacting thePC board 16. A smaller diameter cylindrical shaft 38 extends from thecylindrical portion 34 in axial alignment therewith to pass through thebore 32 and terminate in an end 40 having a square cross-sectionparticularly adapted for attachment of an electrical conductor theretoby a so-called Wire Wrap process. An enlarged portion 42 on the shaft 38contacts the outer ends of fingers 44 formed into the tube 30 to preventlongitudinal movement out of the tube 30 of the head 36 beyond apre-established maximum point. A spring 46 is under compression betweenshoulder 48 and the inside of the fingers 44 to bias the assembly to itsouter position. Since the electrical connection to the probe (not shown)is directly attached to the end 40, there is no danger of loss ofelectrical continuity as the probe 28 is moved in and out of bore 32. Onthe negative side, should it be desired to reconfigure the probe 28 orreplace it, the task is not a simple one.

A test probe of the type shown in U.S. Pat. No. 4,168,873 is shown inFIG. 4 and generally indicated at 50. Probe 50 comprises a socket tube52 which is slid into a pair of aligned bores 32 in the fixture 20.Electrical connection is provided to the tube 52 by pressing theelectrical conductor 54 into a pair of jaws 56 in connector member 58,which is crimped into the one end of the socket tube 52. A tubularsleeve 60 is fitted into the end of the socket tube 52 adjacent theconnector member 58. The head 36 is mounted on a shaft 62 having acylindrical portion 64 sized to slidably fit within the tube 52.Correspondingly, the inner end of the shaft 62 is sized to slidably fitwithin the tubular sleeve 60. Spring 46 is disposed between the tubularsleeve 60 and the cylindrical portion 64 to provide the outward biasingforce on the head 36. Stop member 66 is provided to prevent the shaft 62from moving outward past its previously selected maximum point. Withthis embodiment, the socket tube 52 can be easily removed from the bores32 as necessary. To release the electrical conductor 54, however, itmust be pried from between the jaws 56. Electrical contact between themoving shaft 62 and the electrical conductor 54 is hoped for by contactbetween the cylindrical portion 64 and the inside of the socket tube 52in combination with contact between the inner end of the shaft 62 andthe inside of the tubular sleeve 60.

Turning now to FIG. 5, a probe, generally indicated as 68, is shownaccording to the teachings of U.S. Pat. No. 4,461,993. In thisparticular case, a tube 70 is mounted within the bore 32 of fixture 20.The head 36 comprises the end of a cylindrical shaft 72 slidably mountedwithin a tube 74 having its opposite end 76 of a reduced diameteradapted to snugly fit within the tube 70. The electrical conductor (notshown) is electrically connected to the tube 70 in any manner desiredand electrical connection is made between the tubes 70, 74 by their snugfit together in electrical contact. Shaft 72 has a reduced diameterportion 78 and a bulbous end 80. The tube 74 is crimped as at 82 toprevent the shaft 72 from moving out of tube 74 beyond its desiredextension limit. A spring 46 is positioned within the tube 74 betweenthe portion 78 and the bulbous end 80 to urge the head 36 to itsextended position. While tube 74 and its assembled components can beeasily removed from the tube 70 without disconnection of the electricalconductor (not shown), once again, electrical connection between thetube 74 and shaft 72 is hoped for by the sliding contact of the shaft 72on the inside of tube 74, sliding contact of the bulbous end 80 withintube 74, and electrical contact by the spring 46 between the tube 74 andend 80.

The prior art configurations of FIGS. 6-8 are variations that can beapplied to the embodiments of FIGS. 4 and 5 in an attempt to provide agreater probability that electrical contact will be provided between theouter tube and the shaft sliding therein containing the head 36. FIG. 6corresponds to the teaching from U.S. Pat. No. 4,050,762. In this case,a spring member 82 having fingers 84 is mounted to the end of the movingshaft 86 to press against the inside walls of the tube 88.

FIG. 7 shows the teachings of U.S. Pat. No. 3,753,103, wherein a contactmember 90 is connected to the end of sliding shaft 92 by a leaf spring94 providing a biasing couple tending to urge the contact member 90against the inside walls of the tube 96.

Finally, in FIG. 8, which corresponds to U.S. Pat. No. 4,397,519, theinner end 98 of shaft 100 is angled at surface 102 and the biasing forceof the spring 46 is applied through a ball 104 against angled surface102 which results in a biasing force vector from spring 46 also beingprovided against end 98 towards the inner side wall of the tube 105.

All the above-described prior art probes, many of which remain merepaper proposals, and their manner of connection to the electricalconductor connected thereto have one or more problems relating tomechanical and electrical effectiveness and reliability, and cost, easeof manufacture and ease of use. Especially as the center-to-centerspacing between probes in a multi-probe fixture of necessity becomessmaller and smaller as boards to be tested become corespondinglysmaller, there is a requirement for a probe which is simple tomanufacture because of few and simple parts, while, at the same time,being easily replaceable and completely reliable in its electricalconductivity.

Wherefore, it is the object of the present invention to provide anelectrical circuit test probe and connector combining these features.

SUMMARY

The foregoing objects have been met by an electrical circuit test probefor mounting in a test fixture bore according to the present invention,comprising a cylindrical socket tube of an electrically conductivematerial adapted to snugly fit within the fixture bore, the socket tubeincluding means for electrical connection to an electrical conductor onone end and being open on its opposite end; a plunger member of anelectrically conductive material having a first cylindrical portionadapted to slide fit into the open opposite end and terminating on itsouter end in a head for contacting circuits to be tested, the inner endof the first cylindrical portion having a second cylindrical portionextending therefrom along a common longitudinal axis and being of asmaller diameter to form a shoulder where the two cylindrical portionsjoin one another, the inner end of the second cylindrical member havinga resilient wand extending therefrom at an angular offset to the commonlongitudinal axis, the wand terminating in a bulbous contact member of adiameter substantially the same as the second cylindrical portion; acylindrical plunger tube of an electrically conductive material disposedwithin the socket tube and in electrical contact therewith adjacent theelectrical connecting means with the contact member disposed therein andwith the wand passing through an opening through which the contactmember cannot pass; and a compression spring disposed between theshoulder and the plunger tube whereby a bias force is created on theplunger member towards its extended position and the contact member ismaintained in good electrical contact with the inner surface of theplunger tube by the biasing force from the offset of the wand as theplunger member is moved into and out of the socket tube.

In the preferred embodiment, the electrical connecting means is areleasable connector which plugs into the end of the socket tube andcomprises a cylindrical collet member of an electrically conductivematerial and having jaws at one end facing into a central bore, themember being adapted to have an electrical conductor disposed within thebore and between facing surfaces of the jaws; and, a plug housing of anelectrically conductive material and adapted for electrical connectionto a circuit, the plug housing having a first cylindrical portion withan opening on one end for receiving the collet member and being of aninternal diameter to provide a slide fit for the jaws, the firstcylindrical portion being connected to a second cylindrical portion by asmoothly tapered portion, the second cylindrical portion having aninternal diameter sized such that as the collet member with anelectrical conductor disposed therein is slid into the secondcylindrical portion through the tapered portion, the jaws are squeezedtogether with a pressure to cause the jaws to bite into the conductor,the material of the collet member adjacent the jaws subsequently beingplastically deformed to facilitate further sliding of the collet memberinto said plug housing to maintain the jaws in electrical connectionwith the conductor.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified drawing of a prior art testing techniqueemploying large size hand-held probes.

FIG. 2 is a simplified drawing of a prior art multi-probe test fixtureas wherein the present invention is applicable.

FIGS. 3-8 are drawings from prior art patents showing prior art probes.

FIG. 9 is an end view of the collet member of the present invention.

FIG. 10 is a sectional elevation along section line A--A of FIG. 9.

FIG. 11 is a plan view through the end of the collet member seen in thedirection of arrow X in FIG. 9.

FIG. 12 is a cut-away view through the plug housing of the presentinvention.

FIG. 13 is a cut-away view of the electrical connector of the presentinvention being assembled, but prior to the conductor being gripped.

FIG. 14 is a cut-away view showing the connector of FIG. 13 in its fullyinserted position.

FIG. 15 is a picture of the plunger of the present invention.

FIG. 16 is a cut-away view through the plunger tube of the presentinvention.

FIG. 17 is a picture of the socket tube of the present invention.

FIG. 18 is a cut-away view through the probe and connector of thepresent invention in its fully assembled state.

FIG. 19 is a cut-away view through the plunger tube portion of FIG. 18with the plunger in its fully retracted position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Turning first to FIGS. 9-14, the novel electrical connector portion ofthe present invention will first be shown and described. FIGS. 9-11 showa conductive metallic collet member, generally indicated as 106 adaptedto fit over the end of an electrical conductor in a manner to bedescribed shortly. Collet member 106 is generally cylindrical in shapeand has an opening 108 into which the electrical conductor 110 can beinserted in the manner shown in FIG. 13. In use, the insulation 112 isstripped from the end of the conductor 110 exposing a portion 114 of theconducting wire contained therein. The collet member 106 has a firstcylindrical portion 116 having an internal diameter adapted to fit aboutthe insulation 112 and a second cylindrical portion 118 of a smallerdiameter adapted to fit about the conducting wire 114. The two portions116, 118 form a shoulder 120 at their point of junction, which preventsinsertion of the conductor 110 past its optimal point. The ends of thesecond cylindrical portion 118 are slit at 122 and curved inwardly toform two facing jaws at 124. Each jaw 124 has a neck portion 125connecting it to the remainder of the cylindrical portion. The generallycylindrical collet member 106 has parallel diametrically opposite flatsurfaces 126 formed in the region of and normal to slits 122 to reducethe thickness of material in the collet 106 laterally of the jaws 124.In the preferred embodiment as manufactured by the assignee of thepresent invention, the collet member 106 is of brass tubing per CDA 330,being three-quarter hard. A pair of raised ridges 127 are providedcircumferentially about the exterior of the collet member 106 in spacedrelationship to form a groove 128 adjacent the end with the opening 108.The purpose thereof will be discussed shortly.

It will be appreciated that while two jaws have been described in thispreferred embodiment, three, four or more jaws might be formed and usedin the collet member 106.

Turning now to FIG. 12, the plug housing of the present invention isshown generally indicated as 130. Plug housing 130 is of nickel silverper CDA 752 with its internal surface clad with gold alloy and iscomprised of a first cylindrical portion 132 into which the colletmember 106 can be inserted in a sliding fit as shown in FIG. 13. Plughousing 130 further comprises a second cylindrical portion 134 of asmaller diameter with the two portions 132, 134 being interconnected bya tapered portion 136 smoothly tapering as shown by the angle at 138between the portions 132, 134. The plug housing 130 past the secondcylindrical portion 134 constricts to a closed end 140 of yet a smallerdiameter and forming a shoulder 142 on the exterior where end 140 meetscylindrical portion 134. The end 140 is provided to allow the plughousing 130 to be mounted in the outer end of an electrical circuit testprobe of the present invention, to be described shortly. Othervariations could, of course, be provided within the scope and spirit ofthe present invention. As will be recognized by those skilled in theart, all that is required is that the plug housing 130 be provided withsome means for electrically connecting it to the circuit to which it isgoing to provide releasable electrical access.

Turning now to FIG. 13, it can be seen that in use, the strippedelectrical conductor 110 is inserted into the open end 108 of the colletmember 106 with the insulation 112 abutting against shoulder 120 and theconducting wire 114 passing between the facing jaws 124. A heatshrinkable cylindrical member 144 of electrically insulating material isdisposed around the member 106 between the raised ridges 127 and shrunkinto the groove 128 to thereby grip the groove. Member 144 has an outerdiameter greater than the outer diameter of the probe and connectorthereby to provide electrical separation between adjacent probes in atest fixture. The jaws 124 of the collet member 106 are inserted intothe plug housing 130 as shown in FIG. 13 and then the entire member 106is forced into the plug housing 130 until the innermost raised ridge 126abuts the outer end of the plug housing 130. In that movement, the jaws124 are slid along the tapering portion 136 and into the reduceddiameter second cylindrical portion 134. In so doing the jaws are forcedto bite into wire 114 to produce electrically conductive grippingcommunication therewith with further insertion causing the neckedportions 125 to be stressed sufficiently for them to plastically deformto maintain the facing jaws 124 in intimate (biting) electrical contactwith the conducting wire 114 while permitting complete insertion of thecollet member 106 into plug housing 130. Moreover, the jaws 124 aremoved away from the tapering portion 136 and well into the secondcylindrical portion 134 such that their outward force tends to hold thecollet member 106 within the plug housing 130.

Turning now to FIGS. 15-19, the electrical circuit test probe portion ofthe present invention will be shown and described with particularity.

Turning first to FIG. 15, the plunger thereof is generally indicated as146. Plunger 146 comprises a first cylindrical portion 148 (in someembodiments may have a portion thereof, intermediate its ends, relievedto form two spaced cylindrical surfaces which together correspond toportion 148) having an electrical contact 36 at one end. Extending fromthe opposite end is a second cylindrical portion 152 of a smallerdiameter and in alignment with a common longitudinal axis 154. Extendingfrom the other end of the portion 152 is a wand portion 156 of yet asmaller diameter and terminating in a bulbous contact portion 158 of adiameter substantially identical to that of the second cylindricalportion 152. The plunger is of heat treated beryllimum copper 33-25 orM25, being one-half hard per CDA 173 and plated with 50 micro-inches ofgold over 100 micro-inches of nickel. The wand portion 156 in the testedembodiment is 0.0092 inches in diameter and is offset from the commonlongitudinal axis 154 a distance of 0.010 inches in a length of 0.18inches. As thus sized and constructed, it can be realized that the wandportion 156 acts as a flexible finger with the contact portion 158 atthe end thereof.

Turning now to FIG. 16, the plunger tube is indicated as 160. Plungertube 160 has an internal diameter sized to be a loose slide fit for thecontact portion 158 of plunger 146. The plunger tube in the testedembodiment is CDA #725 clad on both sides with gold alloy.

Turning now to FIG. 17, the socket tube of the present invention isgenerally indicated as 162. The socket tube 162 in the tested embodimentis of beryllium copper alloy clad with a silver alloy on the internalsurfaces. The sizing and interaction of the various parts will beunderstood from the description which follows with respect to FIG. 18.First, end 164 of tube 162 is sized to receive end 140 of plug housing130. When end 140 is inserted into end 164 and the tube crimpedtogether, a secure point of mechanical and electrical contact to sockettube 162 is provided. Socket tube 162 is indented at 166. The outerdiameter of plunger tube 160 is sized such that it can be slid intosocket tube 162 and be held in place adjacent the ends 140, 164 by theinterference fit pressure of the indentation 166. Prior to suchinsertion, however, bulbous end 158 is inserted into the plunger tube160 and the end at 168 crimped about the wand 156 to a size that allowsthe wand 156 to easily slide therethrough, but being of an opening sizeinsufficient to allow the bulbous end portion 158 to pass backtherethrough. Accordingly, contact portion 158 is trapped within theplunger tube 160. Again, prior to such insertion and crimping however,compression spring 170 is slid over the wand 156 and second cylindricalportion 152 into abutment with the shoulder 172 where the secondcylindrical portion 152 joins the first cylindrical portion 148. Then,as the plunger tube is positioned and crimped, the spring 170 is trappedbetween the shoulder 172 and the end 168 of the plunger tube 160.Thereafter, the plunger tube 160 is positioned within the socket tube162 as shown in FIG. 18, which holds the entire probe assembly,generally indicated as 174, in assembled condition. The probe assembly174 can be disposed within a bore 32 on a fixture 20 and is held inplace by the snug gripping action of one or more raised areas 176 on theouter periphery of the tube 162. These raised areas 176 are preferablydisposed in a portion of the socket tube 162 remote from the innermostprojection, in use, of the first cylindrical portion 148 into the sockettube.

As the plunger tube 160 is slid into the socket tube 162 followed by thecylindrical portion 148 of the plunger 146, the wand 156, of necessity,is straightened out from its offset position, thus creating a biasingforce by the contact portion 158 against the interior precious metalclad surface of the plunger tube 160. Thus, as shown in FIG. 19, thecontact portion 158 is held firmly against the inside of the plungertube 160 throughout its entire length of travel, whereby continuouselectrical contact is assured.

Thus, it can be seen from the foregoing description that the presentinvention has truly met its desired objectives. Construction is simpleand of a minimum of parts, which are all of easily fabricated design.The plunger 146, for example, is of unitary construction. The electricalconnector is easily removed, while, at the same time, being of smallsize and affording a firm electrical grip. In the tested embodiment bythe assignee of the present invention, center-to-center dimensions of aslittle as 0.050 inches have been easily obtained with reliable resultsand ease of use in a probe capable of a longer stroke than prior artprobes coupled with higher spring generated contact pressures to ensurereliable electrical contact with PCB's being tested.

We claim:
 1. An electrical circuit test probe for mounting in a mountinghole in a test fixture board comprising:(a) a cylindrical socket tube ofan electrically conductive material sized to snugly fit within themounting hole of the fixture board, said socket tube including means forelectrically connecting to an electrical conductor on one end, and beingopen on its opposite end; (b) a longitudinal plunger member of anelectrically conductive material having inner and outer ends and a firstcylindrical portion slide fitted into said open opposite end andterminating on said outer end in an electrical contact for contactingcircuits to be tested, said inner end of said first cylindrical portionbeing disposed within said socket tube and having a second cylindricalportion extending therefrom along a common longitudinal axis and beingof a smaller diameter to form a shoulder where said two cylindricalportions join one another, said inner end of said second cylindricalmember having a resiliently flexible wand extending therefrom at anangular offset to said common longitudinal axis, said wand terminatingin a bulbous contact member of a diameter substantially the same as saidsecond cylindrical portion; (c) a cylindrical plunger tube of anelectrically conductive material disposed within said socket tube and inelectrical contact therewith adjacent said electrical connecting meanswith said contact member disposed therein and with said wand passingthrough an opening through which said contact member cannot pass; and,(d) a compression spring disposed between said shoulder and said plungertube to create a bias force on said plunger member towards its extendedposition whereby said contact member is maintained in good electricalcontact with the inner surface of said plunger tube by said biasingforce from said offset of said wand as said plunger member is moved intoand out of said socket tube.
 2. The test probe of claim 1 wherein:saidmeans for electrically connecting said probe is a releasable connectorwhich plugs into said end of said socket tube.
 3. In an electricalcircuit test probe for mounting in a test fixture, including a sockettube with a cylindrical spring loaded plunger therein having inner andouter ends and an electrical contact on said outer end, the improvementfor assuring electrical contact between the inner surface of the socketand the plunger as the plunger moves longitudinally comprising;(a) theplunger having a first cylindrical portion and having a resilientlyflexible wand extending into the socket tube therefrom at an angularoffset to the longitudinal axis of said plunger, said wand terminatingin a bulbous contact member; (b) a cylindrical plunger tube of anelectrically conductive material disposed within the socket tube and inelectrical contact with the inner surface thereof and with said bulbouscontact member disposed therein and with said wand passing through anopening through which said bulbous contact member cannot pass, wherebybulbous contact member is maintained in good electrical contact withsaid plunger tube and therethrough with the inner surface of saidplunger tube by the biasing force from said offset of said wand as theplunger is moved into and out of the socket tube and, said cylindricalspring being disposed in compression between said plunger tube and ashoulder on said first cylindrical portion.
 4. The improvement of claim3, and additionally comprising:the socket tube having releasableconnector means for releasably connecting an electrical conductorthereto.
 5. The improvement of claim 4 wherein said releasable connectormeans comprises:(a) a socket formed into the end of the socket tube;and, (b) plug means for electrically gripping the end of an electricalconductor and for releasably fitting into said socket in electricalcontact therewith.
 6. An electrical circuit test probe for mounting in amounting hole in a test fixture board comprising:(a) a cylindricalsocket tube of an electrically conductive material, said socket tubebeing sized to snugly fit in the mounting hole, including means forelectrical connection to an electrical conductor on one end, anddefining an opening at its opposite end; (b) a longitudinal plungermember of an electrically conductive material disposed in said sockettube, having inner and outer ends and a cylindrical portion with alongitudinal axis and sized to be a sliding fit in said socket tubeadjacent said opening, said plunger member terminating at said outer endin an electrical contact for contacting circuits to be tested, saidinner end of said cylindrical portion having a projection extendinglongitudinally from said cylindrical portion and forming therewith anannular shoulder, a portion of said projection terminating in a bulbouscontact member, being resilient and disposed at an angular offset tosaid longitudinal axis; (c) a cylindrical plunger tube of anelectrically conductive material disposed within said socket tube and inelectrical contact therewith adjacent said electrical connecting meanswith said bulbous contact member slidably disposed therein to contact aninner surface thereof and with said projection passing through anopening in said plunger tube through which said bulbous contact membercannot pass; and, (d) a coil spring disposed in compression between saidshoulder and said plunger tube; (e) said bulbous contact member beingmaintained in electrical contact with said inner surface of said plungertube by the biasing force from said offset of said projection as saidplunger slides therein.
 7. An electrical circuit test probe for mountingin a test fixture board comprising:(a) a cylindrical socket tube of anelectrically conductive material adapted to snugly fit within thefixture bore, said socket tube including means for electricallyconnecting to an electrical conductor on one end, and being open on itsopposite end; (b) a plunger member of an electrically conductivematerial having a first cylindrical portion adapted to slide fit intosaid open opposite end and terminating on its outer end in an electricalcontact for contacting circuits to be tested, the inner end of saidfirst cylindrical portion having a second cylindrical portion extendingtherefrom along a common longitudinal axis and being of a smallerdiameter to form a shoulder where said two cylindrical portions join oneanother, the inner end of said second cylindrical member having a wandextending therefrom, said wand terminating in a bulbous contact memberof a diameter substantially the same as said second cylindrical portion;(c) a cylindrical plunger tube of an electrically conductive materialdisposed within said socket tube and in electrical contact therewithadjacent said electrical connecting means with said contact memberdisposed therein and with said wand passing through an opening throughwhich said bulbous contact member cannot pass; and, (d) an electricallyconductive compression spring disposed between said shoulder and saidplunger tube whereby a bias force is created on said plunger membertowards its extended position.
 8. An electrical circuit test probe formounting in a test fixture board comprising:(a) a cylindrical sockettube of an electrically conductive material said socket tube includingmeans for electrical connection to an electrical conductor on one end,and defining an opening at its opposite end; (b) a plunger member of anelectrically conductive material having a cylindrical portion defining alongitudinal axis and adapted to have a sliding fit in said socket tubeadjacent said opening and terminating at its outer end in an electricalcontact for contacting circuits to be tested, the inner end of saidcylindrical portion having a projection extending generallylongitudinally of said cylindrical portion and forming therewith anannular shoulder, a portion of said projection terminating in a bulbouscontact member; (c) a cylindrical plunger tube of an electricallyconductive material disposed within said socket tube in electricalcontact therewith adjacent said electrical connecting means with saidbulbous contact member slidably disposed therein and with saidprojection passing through an opening in said plunger tube through whichsaid bulbous contact member cannot pass; and, (d) a coil spring of anelectrically conductive material disposed in compression between saidshoulder and said plunger tube.